Photoelectric tube



June 1939- H. H. GEFFCKEN ET AL 2,161,859

PHOTOELECTRIC TUBE Original Filed Nov. 16, 1932 INVENTOR. einrich )(Aiefficken Jt'ana gl-aizc fer ATTORNEY.

Patented June 13, 1939 UNITED STATES PATENT OFFICE PHOTOELE CTRIC TUBE Heinrich H. Geffcken, Berlin, and Hans R. Richter, Leipzig, Germany this application June 17, 1936, Serial No. 85,650.

In Germany February 4, 1932 3 Claims.

This application is a division carved out from U. S. application Serial No. 643,163 filed November 16, 1932 now U. S. Patent No. 2,055,487.

Our invention relates to novel photoelectric tubes and more particularly to the arrangement and construction of such tubes.

In the operation of photoelectric tubes of both the high vacuum and gaseous type, it is essential that the emission current passing through the tube faithfully follows the light fluctuations on the cathode or photosensitive surface. Applicants have discovered that by making full use of all the avilable space within the tube, a much larger current output and a more sensitive operation can be obtained. Accordingly, it is an object of our invention to provide a photoelectric tube in which substantially all the inner space of the tube is used for current conduction and wherein substantially no obstructions exist impeding the proper flow of space current within the evacuated space of the tube.

A further object is to provide a phototube construction which is substantially free from socalled microphonic effects caused by mechanical vibration of the tube.

Another object is to provide a gas filled phototube of the character described in which destruction of the light sensitive cathode by ion bombardment is substantially prevented.

There are other objects of our invention which, together with the foregoing will appear in the detailed description which is to follow in connection with the drawing, in which Figure 1 illustrates in perspective view a phototube constructed according to the invention. Figures 2-4 are horizontal cross-sectional views of a tube according to Figure l and showing several embodiments of arranging of the electrodes, Figure 5 is a further cross-sectional view of a tube of the type according to Figure 1 cmbodying a neutralizing electrode to eliminate ion bombardment of the photosensitive cathode. Figure 6 is a front view, and Figures 7 and 8 are cross-sectional side views of a modified tube construction according to the invention.

Similar reference numerals identify similar parts in the different views of the drawing.

The invention herein shown is characterized by the fact that the tube electrodes of extensive surface are closely adapted to the inner wall surfaces of the tube vessel and that their interior space is kept free from any members between them which might hinder the path of the electrons. It was observed that in this way the smallest space charge disturbances were obtained and, in

addition, the possibility of undisturbed attainment of unusually high ionization amplification factor. These conditions become most advantageous if the tube is given the usual well known oval cross-section. Moreover, there are a number of advantages in the invented tubes especially important for light-sound reproduction.

An example of a phototube construction according to the invention is shown in Figure 1. Inside the tube vessel I, equipped with a base 2, are mounted the two curved electrode plates 3 and 4 which are closely adapted to opposite inner wall surfaces of the tube. The tube vessel has preferably an oval cross-section in which the longer axis of this cross-section stands perpendicular to the direction of incident controlling light rays. When a voltage is applied to the electrodes, an electrical field is set up between the surfaces 3 and 4 of very small density of lines of force, which thereby traverses a very large volume of gas in comparison to the available space. The electrode plate 3 is silvered in the usual way, then oxidized, and finally coated with a light sensitive substance such as caesium at higher temperature. It serves as photosensitive cathode. It is understood, however, that the light sensitive surface may be produced in any other known manner. The electrode plate 4 is provided with I the cut-out window 5 which permits entrance of light to the cathode 3 in the direction of the arrows 6. To support the electrodes firmly against each other, the glass bridges l are provided, of which the two upper ones are joined together and propped against the vessel by means of the strut 8. Through this, sensitivity to vibration of the tube is greatly lowered.

Tubes of the described type having an appropriate gas filling, at an extinction potential of about 180 volts, can reach sensitivities up to 150 microamperes per lumen and more, using an operating potential of 100 volts; and even up to 500 microamperes per lumen and more with operating potentials of above volts. Operation at the latter high potentials can be maintained for hundreds of operating hours without injury to the tube.

Instead of mounting both electrodes free, as in Figure 1, it is found useful, in order to improve on the utilization of space, to apply one or both electrodes directly on the glass wall, in form of metallic deposits. The cross-section of such a tube is shown in Figure 2. The oval tube vessel 9 carries in its interior on one side, the cathode metal coating I 2 covered with a photosensitive coating, and on the other side, the anodic metal coating 13 with the window It cut out. With tubes of this type still better results are obtained than with that described in Figure 1 because of the absence of all wall charges. In addition, they are entirely non-microphonic in themselves. On the other hand, to attain and maintain perfect insulation, all traces of superfluous alkali metal should be expelled with extreme care.

Because of this requirement, it is more useful to move at least one of the electrodes away from the wall, Best suited for this is the anode, whereby it becomes possible without too great an increase in capacity, to provide the tube with an outer conductive coating. The latteris connected to ground or the cathode usually grounded in light-sound or other photoelectric apparatus and acts to keep away external capacitative or high frequency effects from the tube. An example of such a tube is shown in Figure 3. Inside the tube vessel 9 is the photosensitive cathode-l2 again directly applied as a metallic coating to one half of the glass inner wall,'while a plate IS with window l6 serves as anode. The latter is set oiI from the tube wall a few millimeters, .and is carried on the braces l8, and propped against the tube vessel. The whole tube except for the window I6 is covered with a metallic coating ll produced by spraying or thelike, such as shown in Figure 6 to be described later. Tubes of this type show a capacity of only 15 to centimeters according to the GCS unit'system'in spite of their complete shielding.

In certain cases, however, it proved more beneficial to set not the anode, but the cathode back from the tube wall. This is especially successful for example, when, to achieve special ,spectral distribution of sensitivity .for reproducing color film, a method 'of producing the light sensitive coating is called for, which requires a massive supporting sheet. In that case, the choice of a tube-form as in Figure 4 is advisable. In the tube vessel 9, the anode l3 in the form of a-metallic wall, coating, is applied to one side of the inner wall with the window, left free to permit the passage of light. Opposite the anode, on the other side, the supporting plate I9 is mounted coated with the photosensitive substance. So far as in such tube it is satisfactory to use a cathode oxidized in the course of manufacture by glow discharge, it is advisable to set the back surface of the supporting plate I9 less than 4 micrometers away from the tube wall, so that during thisoxidation the glow discharge does not intrude into the narrow space thus formed. Thus is prevented the formation of a reservoir of absorbed alkali metal, from which free alkali metal could again be liberated.

It is possible to increase still further the sensitivity, rapidity of response'and length of life 'the'wall at points '22 and 23.

of the tubes made according to our invention by using the dish-shaped electrode referred to, heretofore used as anode, not as anode but as negatively biased auxiliary electrode, and by providing in addition to it and the cathode, a speauxiliary electrode is formed as a metallic cover on the opposite wall surface of the tube. In practice, the cathode I9 is, for example, connected with zero potential, the anode 24 with plus 100 volts, and the auxiliary electrode with minus 20 volts. As soon as light enters the tube in the direction of the arrows through window 26 electrons are liberated from the photosensitive coating l9, fly through the anode 24, and give" rise to positive ions in the space between the latter and theauxiliary electrode 25. After this energy is dissipated they travel back to anode 24, while the generated ions discharge on the.

auxiliary electrode 25 which is to'say, they are kept away from the photo-sensitive coating I9. 7

In order to be able to give a sufficient negative bias without the danger of a discharge-breakdown at the auxiliary electrode 25, it is well to provide the auxiliary electrode 25 with a coating .of higher cathode drop than the cathode Since the drop of the photosensitive coating I9. cathode drop of the photosensitive coatings is very low, it is merely necessary for'this purpose to keep the auxiliary electrode 25 free of alkalicoatings or to coat it with graphite, molybdenum or the like. As mentioned in the parent application, it proves especially useful to coat the auxiliary electrode with a semi-conductor, such as suitably thick iron oxide, distilled selenium, etc. This prevents the breakdown of an independent discharge between the electrodes Y25 and 24, up to very high potentials.

In such a tube we arrive at a practically complete liberation of the photosensitive coating from the destructive action of the ions, thereby attaining an increased life span of the tube, and moreover we make it possible to use coatings of such complicated structure and such high emission as have heretofore been only practicable in high vacuum tubes, because in gas-filled photoelectric tubes, 'up to the present invention, they have. immediately been disintegrated by ion;

bombardment.

By means of the invention, the disadvantage j of capacity eifects due to vibrations of the tube is avoided by covering the tube with a metal cloak or shield tightly bound to the tube, ex-

cept for the window arranged for the entrance of light and for the necessary insulation paths. This cloak is grounded and for this purpose preferably from the beginning on, is securely connected conductively to the tube electrode, generally cathode, which is to be grounded. In

as a whole with respect to surrounding objects In the case of tubes with movable inner members, such as anodes, plate cathodes orthe like, the grounded metallic outer coating produces no more than a shielding against undesiredhigh frequency effects. In fact it is more likely to increase than to reduce the microphonic ,eifect. The aim of our invention can be successfully attained when all the electrodes, on the one side; and the grounded outer covering on the other, are arranged to be wholly immovable; that is, arranged perhaps 'onboth sides of one and the same glass wall in the form of thin coverings. r

'The grounded metal covering can be built-up in various ways. effect is obtained by using a thin metal conductive coating, produced by silvering, spraying or.

the like, and lying directly on the .glass wall. If

The very smallest microphonic special stress is laid on lowered capacity, it is advisable to push a metal tube over the tube and pour cement between.

An example of the first-mentioned kind is shown in Figures 6 and 7 in side view and crosssection (across line l'l) respectively. The elongated, flattened tube Vessel 21 with end caps 28 and 29, carries in its interior, the coatings 30 and 3| applied directly to the glass wall as thin coverings, for example, as silvering; of these coatings, 30 is used as photosensitive cathode and covered for this purpose with a suitable light sensitive substance, and substance 3| as anode. The window 32 is provided for entrance of light and 33 is the conductive coating of the invention; in the present case applied directly to the outer wall of the tube. This is indicated in Figure 6 by shading. As may be seen from Figure 6, it is connected with the cathode lead cap 29, while on the left side of the tube it does not quite reach to the anode cap 28. To protect the cover 33 from corrosion and mechanical injury, it is suitably lacquered.

A tube of different lower capacity design is shown in cross-section in Figure 8. The tube vessel 21 again has oval cross-section, while the metal cloak 34 (sheet metal tube) is round. The space between is filled up with a suitable cement mass 35 which establishes rigid connection between tube and cloak, and maintains the cloak upright. Tubes of this kind are likewise practically completely non-microphonic and can be rigidly built into the sound or other light sensitive apparatus. They present a substantially reduced capacity as compared with the tubes shown in Figures 6 and '7. r

Although we have described preferred forms of our invention, it will be obvious to those skilled in the art that it may take other forms which come within its scope, and we do not wish to be limited by the illustrations herein given except as set forth in the appended claims.

We claim:

1. A photoelectric tube comprising a cylindrical envelope of oval cross-section, a gaseous atmosphere therein, curved cathode and anode electrodes substantially conforming to and mounted adjacent to the wall portions of opposite halves into which said envelope is divided by the longer axis of its cross-section, said anode having a central window for passing controlling light rays unto said cathode, and a coating of photoelectrically active material on the inner surface of said cathode.

2. A photoelectric tube comprising a cylindrical envelope of oval cross-section, a gaseous atmosphere therein, curved cathode and anode electrodes substantially conforming to and. mounted adjacent to the wall portions of opposite halves into which said envelope is divided by the longer axis of its cross-section, said anode having a central window for passing controlling light rays unto said cathode, a coating of photoelectrically active material on the inside surface of said'cathode, and a metal coating covering the outside of said vessel excepting the area overlying said window.

3. A photoelectric tube comprising a pair of curved large surface electrodes mounted with their concave sides facing each other, a coating of photoelectrically active material on the inside surface of one of said electrodesacting as a cathode, the other electrode acting as an anode and having a central window therein, a transparent vessel of insulating material conforming to and closely surrounding the outer surfaces of said electrodes, and a gaseous atmosphere within said vessel.

HEINRICH H. GEFFCKEN. HANS R. RICHTER. 

